1. Field
The present disclosure is directed to a method and apparatus for handover in a wireless communication system. More particularly, the present disclosure is directed to handover of a mobile station from a source base station to a target base station.
2. Description of Related Art
Presently, wireless network systems can use multiple base stations to provide coverage for mobile stations, such as cellular phones. When traveling in a wireless network system, the mobile station can be handed off from one base station to another depending on which one is best suited for the mobile station. For example, when the mobile station is using a first base station and then moves closer to a second base station, the first base station can hand the mobile station over to the second base station if it determines the second base station will provide a better signal for the mobile station. Such a procedure can be used in current systems and Fourth Generation (4G) systems, such as the Third Generation Partnership Project Long Term Evolution (3GPP LTE) system.
Unfortunately, signaling in current networks result in call interruptions when the mobile station is handed over from one base station to another. For example, in current systems, the procedure to acquire uplink synchronization with the target base station requires the mobile station to interrupt its uplink and downlink data transfer through the source base station. Due to delays involved in re-routing un-transmitted packets for transmission via the target base station, such packets may be too late and may simply be discarded, especially for real-time services such as voice. Furthermore, the probability of success of a Random Access Channel (RACH) attempt can vary and, in cases where the mobile station is near a boundary of the source base station and the target base station, it is likely that the RACH procedure will take several attempts. With current interruption times of 10-20 ms the outage criterion is quickly reached and a mobile station that sees a 2% Frame-Error Rate (FER) is considered to be in outage.
Another problem exists for non-contention preambles that are used to avoid RACH transmission conflicts between different mobile stations by distinguishing between different mobile stations. Assigning a non-contention preamble is a very complex task at the target base station. Since the preamble is used on the RACH of the target base station, it has to be picked by the target base station, so that it can assign different preambles to different mobile stations that are attempting handover into the target base station. The target base station has to process handover requests from different source base stations in real-time, and reserve RACH preambles for each request. This reserved preamble has to be communicated to the respective source base stations which then assign them to the respective mobile stations. The time required to deliver the message containing the reserved RACH preamble to the mobile station can vary; thus the target base station has to reserve the RACH preamble for multiple RACH opportunities. The target base station also needs to maintain a timer associated with each assigned preamble and release the preamble into the available pool if the preamble is not used within a certain time limit. While this implementation is necessary to maintain the contention-free property, the likelihood of two or more mobile stations attempting to handover to a target base station at the same RACH opportunity is generally low. Thus, the gains of the contention-free scheme are not commensurate with the complexity of the procedure required to manage the contention-free scheme.
Thus, there is a need for handover procedures that eliminate or minimize the interruption time. There is also a need for handover procedures that are less complex and effectively contention-free.